Method for determining and controlling the chlorine content in a bleach bath



Feb. 26, 1957 F. LOSCHBRANDT 2,733,135

METHOD FOR DETERMINING AND CONTROLLING THE CHLORINE CONTENT IN A BLEACH BATH Filed Jan. 2'7, 1953 3 Shuts-Shut 1 Feb. 26, 1957 F. LUSCHBRANDT METHOD FOR DETERMINING AND CONTROLLING THE CHLORINE CONTENT IN A BLEACH BATH Filed Jan. 27, 1953 3 Shanta-Shoot 2 MILLIVOLT .1 ,2 GRAM CHLORINE PER LITER 1957 F. LGSCHBRANDT 2,783,135

METHQD FOR DETERMINING AND CONTROLLING THE CHLORINE CONTENT IN A BLEACH BATH Filed Jan. 27, 195:5 s Shouts-Sheet :5

I MINUTE 0,/6 7- RESIDUAL LHMRINE' 0, 39 RESIN/M (I/URINE United States Patent METHOD FOR DETERMINING AND CONTROL- Lg; THE CHLORINE CONTENT IN A BLEACH B T Frithjof Liischbrandt, Oslo, Norway, assiguor to Papiriudustriens Forskningsinstitutt, Skoyen, Oslo, Norway Application January 27, 1953, Serial No. 333,463

Claims priority, application Sweden January 30, 1952 1 Claim. (Cl. 23-230) In a continuously running oxidation or reduction process, as for instance, the chlorination stage in the continuous bleaching of chemical pulp, there is a great need of a process for indicating, recording and contingently controlling the content, excess or deficiency, of active chlorine, or generally of oxidizing or reducing agent. To simplify the matter, only the reaction with chlorine during a multistage cellulose bleaching will be mentioned in the following. It will, however, be obvious that a method may be adapted to any stage in the bleaching process where the oxidizing agent is added, and generally to many cases where an oxidation or reduction process is running continuously.

Variations in the consistency of the pulp, brought into the bleachery, and unavoidable variations in the bleachability of this pulp, make frequent adjustments of the chlorine supply necessary, because an excess supply may be directly harmful to the pulp, or give rise to unnecessary great consumption of chlorine, whereas a deficiency of chlorine may result in poor bleaching.

As the bleaching process is carried out, now an excess of chlorine will not manifest itself until the reaction mixture is leaving the reaction vessel, and a deficiency will not be noticed until the end of the reaction process, and then it will be too late to rectify the matter.

A direct measurement of the oxidation or reduction potential of the reaction mixture cannot lead to any adequate result, because this potential not only is dependent on the amount present of the original composition of the reaction agent added, but also on the amount of the reacted part, which in turn is dependent on the total consumption of reaction agent. In such a continuous bleaching of cellulose with chlorine the oxidation potential, accordingly, will be dependent on the amount of the chlorine ions formed, which in their turn will be dependent on the bleachability of the pulp, wherefore the oxidation potential for one and the same excess of active chlorine will not be unambiguous.

The above mentioned difiiculties may, according to the present invention, be avoided, and it will be possible very accurately to determine the chlorine content of the pulp after the addition of chlorine. It will in this way be possible seasonably to adjust the chlorine content, either by decreasing or increasing the supply of chlorine.

The method according to the present invention consists in mixing together in a fixed proportion a representative sample of the reaction mixture and a solution containing a reversible auxiliary red-ox system with an appropriately chosen proportion between the oxidized and the reduced form of the system, and with a suitable normal analyzing vessel.

the reduced forms is approximately linear, for instance in a proportion of reduced to 20 oxidized parts on a mol basis or vice versa. The mixing of the two solutions may, dependent on the apparatus chosen, be made continuously or discontinuously.

The displacement of the potential is determined by a suitably chosen electrode system, and the potential diiference is conveyed to a potentiometer, which may be recording and serve as an impulse for a servo-mechanism, which opens or closes the supply of reaction agent, so that the amount of this is kept on an appropriate level, or within specified limits.

The sample to be analyzed is taken out at a place which according to experience lies so far removed from the place where the chlorine is added that the obtaining of an average sample, representative for the reaction mixture, is assured. This sample, which may be taken out continuously or discontinuously is ridded of fibre and is led to the analyzing apparatus where it is mixed with a solution which in this case is about 0.01 molar in respect of iron ions and where the proportion between the oxidized and the reduced form of the iron originally is as 20 to 80, calculated on mol basis, and where the iron solution is made acid with strong acid. The proportion between the volumes of the two solutions may suitably be chosen as lzl, but other proportions may be more appropriate in other cases.

To carry out the measurement, that is, the analysis, it is proceeded in the manner that an electrode, for instance of bright platinum, is placed in the above mentioned mixture of chlorine solution and iron solution, and that a conducting connection is established with another vessel in which also is placed an electrode of the same kind an which serves as a reference electrode. In this vessel is also part of the iron containing auxiliary solution, but diluted in the samgprpportion as the iron solution in the In the present case the auxiliary solutiOn tH'LKWiII'BQOOS M in respect of total iron ions. According to the content of chlorine of the mixture to be measured, a potential difference between the two electrodes will appear, which is conveyed to a potentiometer. If no chlorine is present, the potential difierence will be practically zero.

As a reference electrode, there may also be applied any other electrode with a constant potential. The potential difference between the electrodes will then generally not be zero at zero content of residual chlorine. The zero potential difference should in this case be compensated by an auxiliary potentiometer before the electrode system is allowed to act on the recording potentiometer.

In the drawing,

Fig. l is a schematic view of an apparatus in accordance with the invention;

Fig. 2 is a schematical view, partly in section, of a measuring device;

Fig. 3 is a fragmentary schematic view, similar to Fig. 1, but embodying a modification;

Fig. 4 is a graph showing the relation between the concentration of active chlorine and the measured potential difierential; and

Fig. 5 is a graph showing the relation of time and concentration, where the time unit is one minute.

An apparatus for the carrying out of the method when this is adapted to the analyzing of chlorine solution from the chlorination stage of a cellulose bleaching is shown in Fig. 1.

A and B are two pipettes which are supplied with fibre-free solution to be analyzed from the chlorinating vessel and the auxiliary solution, respectively, which latter contains the reversible red-ox system, through the pipe lines a and b, provided with valves a and b,. The filled pipettes which in the present case are of equal size,

are simultaneously emptied through the tubes c, d by means of the valves c,, d, into the mixing vessel C, which is provided with a mechanical stirrer not shown. The mixture is conducted through the tube e and the valve e, (which contingently may be superfluous) with an appropriate time lag, so that a good mixture is achieved in the vessel C, to the vessel D where the resulting red-ox potential of the mixture is measured. This is made by means of the electrodes E1 and E2, of which E1 is the measuring electrode, and E2 the reference electrode with constant potential.

The apparatus is supplied with the solutions automatically by controlling the valves a,e,, for instance mechanically or electromagnetically.

In Fig. 2 there is shown a detailed design of a measuring vessel which has been found suitable for the present purpose. The measuring vessel D is at the bottom supplied with the mixture to be measured through the tube e, and is moreover provided with an overflow G to the drain. Near the bottom of the vessel a measuring electrode E1, for instance of bright platinum, is placed. In the vessel there is further placed a glass tube H, onto which a bottom of sintered glass is fused, the glass tube having also a supply tube F for the auxiliary solution, and being provided with another electrode E2. By letting a small amount of the auxiliary solution flow through the tube H and the sintered bottom, the electrode E2 will always be surrounded by the auxiliary solution, and any penetration of reacted solution from the outside is made impossible and consequently also any disturbance from this solution. By placing the electrode E1 near the bottom of the vessel, any disturbance of the potential of this electrode by the solution surrounding the reference electrode is also avoided.

The apparatus now operates in the way that the valve a, at first is opened, and the solution to be measured is allowed to flow through the measuring pipette A and out through the overflow. In this way all previous solution is washed out of the pipette. The valve b, is opened, so that pipette B is filled, and as soon as it is completely filled, the valves 0, and b, are closed. The valves 0, and d, are now opened simultaneously and the solutions in A and B are emptied into the mixing vessel C, where mixing takes place by powerful stirring. As C is placed somewhat higher than an overflow from D, the mixture flows to the vessel D and out through the overflow when the valve 0, is opened. During this phase of the operation. a p9 t ent ial ditferenceismeasured, which, when not of the prescribed value, starts the regulating mechanism. As soon as the pipettes A and B are emptied, the valves 0, and d, are closed automatically, and the whole operation repeats itself.

As the filling and emptying of A and B take place within the course of a few seconds, the potentiometer practically will record the potential difference continuously and also control the chlorine content of the chlorine solution in the same way.

Instead of using adjusted pipettes A and B, which are emptied periodically, adjusted capillary tubes may be employed, which stand under a constant hydrostatic pressure.

Fig. 3 shows such an arrangement where A and B are two tubes, which may be equally long or of an unequal length, and which are provided with overflow tubes and with supply tubes for the two solutions. In the bottom of these tubes are adjusted capillary tubes K1 and K2. From these two capillary tubes, which stand under a constant hydrostatic pressure, an accurately defined amount of solution will flow in the proportion specified. This proportion is determined by the hydrostatic pressure and by the dimensions of the capillary tubes. The solutions are mixed in the mixing vessel and the mixture flows continuously further to the measuring cell. In this way a continuous indicating, recording and control of the chlorine content is obtained.

For the determination of the chlorine in the chlorination fluid an iron solution consisting of Mohrs salt and iron 3-chloride is suitable for the purpose. The chemicals are mixed in a mol proportion of 4: 1, and in a concentration of total iron of 0.01 mol/litre. This ferro-ferrisolution is made 0.1 M in respect of sulphuric acid.

In Fig. 4 there is shown the relation between the concentration of active chlorine in the chlorination fluid and the measured potential difference when chlorinating solution with a varied content of chlorine is mixed with a ferro-ferri-solution of the above mentioned composition and in the porportion 1:1 by volume.

Fig. 5 shows a reproduction of a diagram from an actual run, where the concentration of chlorine is varied intentionally, and where the unit on the time axis is one minute.

As mentioned in the beginning, the method according to the invention may be adapted to the indicating, recording and controlling of any continuously running oxidation and reduction process whatsoever, by choosing a suitable red-ox system with appropriate normal potential and by the appropriate choice of concentration and pH of this system.

I claim:

In a method for indicating, recording and controlling the chlorine content of a bleaching bath in the bleaching of chemical pulp, the steps comprising, mixing representative samples of the bath liquid in accurately defined proportion with a control solution of a red-ox system consisting of an aqueous solution of ferro-ferri ions in defined proportion and of suitable acidity, passing the resulting mixture through a measuring cell with an electrode in contact with said mixture and another electrode in contact with a ferro-ferri ion reference solution of predetermined potential applying the resulting potential difference between the mixture and the reference solution on an indicating and recording potentiometer and also applying said difference for controlling the supply of chlorine to said bath.

References Cited in the file of this patent UNITED STATES PATENTS 1,450,023 Edelman Mar. 27, 1923 1,530,833 Keeler Mar. 24, 1925 1,759,996 Parker May 27, 1930 2,076,964 Pomeroy Apr. 13, 1937 2,289,589 Pomeroy July 14, 1942 2,289,610 Wallace July 14, 1942 2,396,934 Wallace Mar. 19, 1946 2,430,895 Tuve et al. Nov. 18, 1947 2,513,562 Holuba July 4, 1950 2,560,317 Wallace July 10, 1951 OTHER REFERENCES Scientific American, May 24, 1919, page 548.

Ind. and Eng. Chem, vol. 20, No. 2, page 214, February 1928.

Kolthoff and Laitinen: Ph and Electro Titrations," 2nd ed. (1948), pages 66, 67; John Wiley and Sons Inc. 

